The invention relates to a photoelectric semiconductor light-detection device having high dynamic range by virtue of a programmable offset signal.
One known method of reducing the sensitivity of photoelectric semiconductor light-detection devices overall, that is to say for all the pixels at the same time, is the so-called electronic shutter, as described, for example, by H. Akimoto, H. Ono, M. Nakai, A. Sato, T. Sakai, M. Maki, M. Hikiba and H. Ando in "Lateral overflow-gate shutter for CCD image sensors", Proc. SPIE, vol. 1656, pp. 550-557 (1992). In this case, an electronic switch is opened only for a fraction of the integration time, that is to say for the so-called exposure time, to integrate the charge carriers produced by photons. The sensitivity of all pixels can thereby be reduced simultaneously by the integration time/exposure time factor.
A method which makes it possible to use this on/off switching technique individually for the individual pixels of an image sensor has been described by S. Chen and R. Ginosar in "Adaptive sensitivity CCD image sensor", Proc. SPIE, vol. 2415, SUM. 303-309 (1995). In this technique, however, all the pixels are driven at high speed and repeatedly during the image acquisition. This technique cannot therefore be used in practice for relatively large image sensors having hundreds of thousands of pixels.
A known technique of subtracting an offset quantity of charge from an integrated quantity of charge, produced by photons, is the so-called "fill-and-spill" method in CCD technology, as described by W. Yang in "Analog CCD processors for image filtering", Proc. SPIE, vol. 1473, SUM. 114-127 (1991). In this case, a potential well is provided, the depth of which corresponds to the offset quantity of charge. Further to this, a potential well is provided, the depth of which corresponds to the quantity of charge produced by photons, and which is therefore somewhat deeper. When the potential wells are flooded with charge carriers, the somewhat deeper potential well retains a quantity of charge which is proportional to the difference in quantity of charge between the offset quantity of charge and the photocharge. This technique works reliably only in the case of a difference in quantity of charge which is in the percent range of the offset quantity of charge.
Another known technique is based on the use of a current-store element in the feedback circuit of a charge amplifier, that is to say of an operational amplifier having a capacitor in the feedback circuit. This has been described by E. R. Fossum and B. Pain in "Infrared Readout Electronics for Space Science Sensors: State of the Art and Future Directions", Proc. SPIE, vol. 2020, pp. 262-285 (1993). Since a current source is connected in parallel with the integration capacitor, the quantity of charge actually integrated is given by the difference between the quantity of charge produced by photons less an offset quantity of charge which is delivered by this current source. This publication describes that by using this technique for infrared sensor technology, effective compensation of the background infrared sensor technology, effective compensation of the background current and of the fixed pattern noise was achieved, albeit with considerable outlay of circuitry. For voltages across the capacitor which are small in comparison with kT/e, that is to say for a few tens of mV, a transistor can no longer be operated in the saturation range, and nonlinearities are to be expected with this technique.